It's so you can use it later in the function easier. You don't need it.
You especially don't need it if you name the object.
There can be - you might have multiple objects called Thing because you want them all to have Thing::blah() functions by default.
Typically in that case you'd declare them with class = Thing;
I remember trying this with various objects and it didn't do anything.
then you did it wrong, because i've done it a lot